The present invention concerns cross-linkers for cross-linkable optical polycarbonates, cross-linked optical polycarbonates, and optical articles obtained therefrom.
In the field of optical polymers polycarbonates are preferred in view of their low light-absorbance in the wavelength area for optical uses (1270-1600 nm), making it possible to obtain optical components with low light loss. Polymeric optical components usually have a multilayer polymeric structure on a substrate with a guiding layer (the core layer) sandwiched between two polymer layers of a lower refractive index (cladding layers). Said layer structure can be formed conveniently by applying the various subsequent layers in the form of a solution, e.g. by means of spin-coating, followed by evaporation of the solvent. However, it was found that in layer-on-layer spin-coating the first polycarbonate layer re-dissolves on applying the next layer onto it. As it is essential for optical components to have a specific layer structure with each layer having a specific refractive index and layer thickness, such re-dissolving of the layers is a serious problem. Because no well-defined refractive index contrast and specific layer thickness can be obtained, the light traveling through the component is not confined within the core layer, resulting in substantial light losses. In EP-0,645,413 NLO (non-linear optical) polycarbonates are described. In order to solve the layer-on-layer spin-coating problem, it is suggested to introduce polyisocyanates or polyepoxides into the polycarbonate as a cross-linker. However, it proved quite hard to make a cross-linkable polycarbonate without detrimentally influencing the properties which determine the applicability of optical polycarbonates in optical components, such as Tg, refractive index, light loss, etc. For instance, it was found that the use of the polyisocyanate Desmodur N.RTM. posed problems due to swelling of the layers. Furthermore, it was found that even after curing the polycarbonate was still partly soluble in the spin-coating solvent. Also, the use of polyepoxides was not optimal. It was found that the layers could not be reproducibly obtained. Some of the layers were sound but others appeared to have cracks.
Earlier solutions to this problem were disclosed in our co-pending patent application PCT/EP96/01101, wherein a novel cross-linker has been disclosed. However, some applications demand considerable amounts of said cross-linker leading to cross-linked polycarbonates which are less suitable for optical application. Therefore a need exists for other cross-linkers having less disturbing optical properties. We have found that specific cross-linkers of the maleimide type can be applied to various types of polycarbonates, improving their properties considerably. Cross-linkers having a maleimide moiety as such are known, for example, the dichloro- and dimethyl-maleimide cross-linkers of German patent applications DE 3443091 and DE 3513715, and the 5-maleimido-isophthaloylchloride as disclosed by Mikroyannidis in J. Polymer Sci., Part A: Polymer Chemistry, 28 (1990), 669, and in German patent application DE 2626832. The use of these compounds for obtaining polycarbonates with advantageous optical properties is not disclosed in any of these prior art references.
In Chemical Abstracts, vol 111, no. 26 (abstract no. 244322) the compound 3,4-dihydroxyphenylmaleimide has been disclosed as an intermediate for the preparation of an amide derivative of maleic acid. No protection for this compound per se is sought.
The present invention provides novel cross-linkers for use in cross-linkable polycarbonates wherein the properties determining the applicability of the cross-linked polycarbonates in optical components are hardly detrimentally affected if at all, and the layers can be applied on top of each other without the first layer being re-dissolved. The novel cross-linkers make it superfluous to use tertiary amines as curing catalyst, which is of considerable advantage because tertiary amines degrade the polycarbonate. Small amounts of peroxide or even no catalyst at all suffice when using the novel cross-linkers. A further advantage is that polycarbonates having nitro groups can also be cross-linked, which is not possible with the known cross-linkers of the acrylate type. Another advantage of the presently claimed cross-linkers is the possibility to cure the polymer by irradiation with UV light, which introduces considerably less strain into the polymer than thermal curing. The novel cross-linkers provide cross-linked optical polycarbonates of excellent thermal stability.